We love a good counterfactual, don’t we? They are a bit of fun, in which we tweak history’s nose by imagining what might have been. Even Edward Gibbon did it in The History of the Decline and Fall of the Roman Empire, when he speculated what might have happened had Frankish ruler Charles Martel not defeated the Moors in 732: “Perhaps the interpretations of the Koran would now be taught in the schools of Oxford, and her pulpits might demonstrate to a circumcised people the sanctity and truth of the revelation of Mohammed.”

That is nose-tweaking with a vengeance – and as Professor Evans points out, it was also a dig at Oxford, where Gibbon had spent “what he called the most idle, and the most unprofitable years of his life”. But Evans disapproves of the current trend for counterfactuals, and arrives like a stern teacher to break up the frolics of naughty schoolchildren. And once his argument hits its stride he makes a very good case.

It is interesting to be reminded that so many of the historians who go in for this kind of thing (Niall Ferguson, Andrew Roberts, Norman Stone, Adam Zamoyski and, earlier, JC Squire, a notable literary figure of the interwar period, along with the contributors to his 1931 volume If It Had Happened Otherwise) are rightwingers. The first chapter of Altered Pasts is called “Wishful Thinking”, and you can begin to feel distinctly queasy when you notice how many counterfactual histories describe a reality in which the British empire never declined or, even worse, where Hitler had won the second world war.

It is notable how many counterfactualists tend to be Eurosceptics, or deeply suspicious of anything that might come to be a federal Europe; in his chapter on counterfactuals in fiction, Evans notes that Robert Harris’s Fatherland – which may be regarded as the modern counterfactual novel par excellence – is steeped in a distrust of Europe. I must admit I missed this aspect of the book, but in my defence, I was just after an undemanding holiday read at the time.

Evans has his sights trained most firmly on Ferguson, who edited the 1997 essay collection Virtual Histories, which reinvigorated the genre. Ferguson, Evans explains, not only shows his hand too clearly in his introduction and own contribution (the Royalists winning the English civil war, etc), but contradicts his own anti-determinism. Counterfactualism’s practitioners prefer great people and events to broad trends, but Ferguson is determinist when it suits him – when it comes to the causes of the first world war, for instance. Evans’s undermining of Ferguson is all the more convincing because he gives Ferguson a good deal of credit and respect before shooting him down. You cannot be a responsible historian, he says, while ignoring the “lengthy chain of causation” that history is about and “the strong degree of arbitrariness in such speculations” in the first place.

The trick, then, is to avoid being too serious about it, and not to let your thinking be guided by the bees in your bonnet. (Catholics imagining a world without the Reformation – that’s another popular train of thought.) Evans cites with great approval the “What If ...” columns written by Dominic Sandbrook for the New Statesman between 2009 and 2011. These were some of my favourite bits of the magazine, in which his counter-historical musings were the basis of hugely amusing gags, such as when the descendants of Oliver Cromwell, “Praise-God” and “Ed” Cromwell contest the leadership of the Labour party; when Sir George Harrison, in a Beatles-less world, becomes a plutocratic curry magnate; and the speculation that ends: “The beret and the polo neck remain essential components of English national dress, pétanque is still our national sport and, above all, everybody loves a mime artist. What a tragedy for England it would have been if Henry V had died young.” That is the way to go about it: not to treat the genre as a way of fantasising about the removal of your grievances. Counterfactuality is not a respectable historical tool, so don’t treat it like one.